fix(agent): harden Alpine OpenRC installation

[Sagit-chu]

Summary

• force Alpine agent installs to use OpenRC rather than systemd
• avoid systemctl calls while cleaning legacy GOST files on OpenRC hosts
• install missing Alpine download dependencies (curl and CA certificates)
• document the Alpine behavior and add a regression test

Root cause

The installer had OpenRC lifecycle support, but the legacy cleanup path still called systemctl unconditionally. Minimal Alpine installations can also lack curl, causing version resolution and binary download to fail after the Bash bootstrap.

Validation

• bash -n install.sh
• bash -n test-install-scripts-proxy.sh
• bash test-install-scripts-proxy.sh
• (cd go-backend && go test ./...)
• Alpine 3.22 container dependency installation and OpenRC detection checks